The present invention relates to an image projection apparatus such as a liquid crystal projector having an exhaust fan.
In image projection apparatuses (hereinafter referred to as projectors), many components that become heat sources are arranged. The heat sources include a light source lamp, a light modulation element such as a liquid crystal panel, optical components such as optical elements, and electrical components such as a power supply ballast and a CPU. Among these components, in particular, the light source lamp generates a large amount of heat which largely affects cooling of other components. Therefore, it is necessary to efficiently exhaust heat from the light source lamp to the outside of the projector.
On the other hand, light leakage from an exhaust outlet provided to the projector causes problems such as glare to a user and lowering of contrast of projected images. Therefore, in the vicinity of the exhaust outlet in the projector, a configuration is desired which prevents the light leakage while allowing a heat-exhausting air to pass smoothly.
Japanese Patent Laid-Open No. 2005-25035 discloses a duct (straight-type duct) in which an inflow opening and an outflow opening are arranged in parallel with each other and a plurality of louvers each having an S-shape is provided thereinside. Using such a duct including the louvers enables a smooth exhaust of an air that has cooled the light source lamp while blocking the leakage light from the light source lamp.
Meanwhile, because of the demand for a further size reduction of projectors, a cooling structure using a bent-type duct such as the one shown in FIG. 8 has been increasingly employed instead of the cooling structure using the straight-type duct such as the one disclosed in Japanese Patent Laid-Open No. 2005-25035.
Referring to FIG. 8, a cooling air W1 is supplied to a light source lamp 101 from a cooling fan (not shown). An air (airflow) WA and an air (airflow) WC that have passed through the inside and outer circumference of the lamp 101 to cool it flow into a duct 127. The duct 127 is a bent-type duct having an inflow opening and an outflow opening formed nonparallel to each other. The airflows WA and WC having flowed into the duct 127 turn into airflows WB and WD whose flow directions are changed by a duct wall surface 127a to be guided toward an exhaust fan 118. The exhaust fan 118 then exhausts the air WB and air WD to the outside of the projector.
However, when such a bent-type duct 127 is used, because of the influence of inertia forces of the airflows WA and WC flowing into the duct 127, unevenness in flow rate of the airflows WB and WD flowing out from the duct 127 toward the exhaust fan 118 is generated. That is, the flow rate of the airflow WB flowing along the duct wall surface 127a becomes higher than that of the airflow WD flowing through a region away from the duct wall surface 127a. 
Such unevenness in flow rate increases noise generated in the exhaust fan (axial flow fan) 118.
FIG. 9 shows a section of the exhaust fan 118. As mentioned above, the airflow WB flowing from the duct 127 toward the exhaust fan 118 has a higher flow rate than that of the airflow WD. When the exhaust fan 118 is rotated, a distal end 118Fa of each blade in the rotating direction of the exhaust fan 118 cuts the respective airflows WB and WD perpendicularly.
In this case, when the distal end 118Fa of the blade finishes cutting the airflow WD having a lower flow rate and begins cutting the airflow WB having a higher flow rate, it hits a side face of the airflow WB, thereby generating wind roar. This wind roar has peaks at a frequency of an integral multiple of “the number of the blades x the rotating speed thereof”, causing noise.
The difference in the flow rate of the airflows WB and WD sucked into the exhaust fan 118 also changes an angle of attack between each airflow and the blade passing through that airflow, whereby airflow burbling (turbulence) is generated on the surface of the blade. This generates turbulence noise and thereby increases the noise, as well as deteriorates the P (pressure)-Q (flow quantity) characteristics of the fan, thereby obstructing the normal operation of the exhaust fan 118.